1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner, by applying an adaptive control theory thereto.
2. Prior Art
There is conventionally known an air-fuel ratio control system for internal combustion engines, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 3-185244, in which an optimal regulator which is one of modern control theories is applied to air-fuel ratio feedback control such that the air-fuel ratio is feedback-controlled based on an output from a linear-output oxygen concentration sensor (LAF sensor) arranged in the exhaust system of the engine, and an optimum feedback gain and state variables which have been set based on a dynamic model representative of the behavior of the engine.
According to the above conventional air-fuel ratio control system, however, no contemplation is made of deterioration of the response characteristic of the air-fuel ratio sensor due to aging or the like, and therefore, when the sensor has become deteriorated to a degree exceeding a degree expected when it was originally designed, an ineffective time (dead time) of the dynamic model representative of the behavior of the engine changes to an unnegligible degree. As a result the controllability of the air-fuel ratio can be extremely degraded.
Further, according to the above conventional air-fuel ratio control system, the optimum feedback gain set based on the dynamic model representative of the behavior of the engine is employed, and therefore, when the response characteristic of the air-fuel ratio sensor and/or other factors which cause changes in the dynamic model representative of the behavior of the engine are deteriorated, the optimum feedback gain can have an inappropriate value to proper feedback control of the air-fuel ratio to be carried out.